Mass spectrometry



July 13, 1948. H. w. wAsHBuRN MASS SPECTROMETRY 2 Sheets-Sheet l Filed Dec. 9, 1943 mmol July 13, 1948 H; 'w. wAsHBuRN MASS SPECTROMETRY 2 Sheets-Sheet 2 Filed Deo. 9, 1943 w w m m nu m m m NS NA wm mw. s m e. m M w m wm m Afferma humanly 13,1948 i uN-rrao s'rAras 'PATENT ori-'ica 2.444,9 Y A t amm w. mbmrmrm., minor to Consolidated Engineering Corpo Pasadena, Calif., a corporation of Calllornia tion,

Application December 9, 1943, Serial No. 513,529

4 claims.' (ci. ase-41.9)

This invention is concerned with mass spectrometers and particularly with spectrometers provided with an ionization chamber and an electron emitting element. such for example as a heated lllament, which provides electrons for bombarding molecules in the ionization chamber to convert them .to ions.

A mass spectrometer is an ion sorting device which may be employed to ionize molecules and separate the resulting ions according to their specillc masses, i. e., according to the ratio of the mass of the ion to the charge which it carries. It may be employed for both qualitative and quantitative'analyses of mixtures, particularly gaseous mixtures. as well as for the determination of isotope abundance ratios and other scien- .tiilc problems. A well known form of mass spectrometer comprises an ionization chamber, means for admitting molecules lto be ionized into the chamber, an electron emitting element, means for propelling the emitted electrons 4against molecules in .the chamber to convert them into ions, an analyzer tube (connected to the ionization chamber) in which an electric or magnetic eld may be established, electrodes disposed in the neighborhood of the electron emitting element and the ionization chamber for impressing a propelling potential on the ions formed in the ionization chamber to force the ions through the analyzer, and a collector upon which the ions impinge and discharge after travel through the ionization chamber.

A sample to be analyzed, say a gaseous mixture, is admitted to .the ionization chamber and there bombarded by Ithe electrons so that ions are formed. These ions are forced by the propelling potential out of the ionization chamber and through the analyzer tube, where (under the action ot the ileld) they are separated into diverglng beams of ions according to their specic mass. each beam comprising a stream of ions havins the same speciilc mass'. By sweeping these beams successively over the collector, or by moving the collector into the paths oi the several beams, a series of charges is collected. Measurements of the amounts of Ithese charges represent the mass spectrum of the gas subjected to ionization. This spectrum is a measure of the amounts of the several kinds oi' ions collected and, under certain operating conditions, can be a measure lo! Athe amounts of the several kinds oi' molecules in the original mixture from which the ions were derived. In short, under proper operating conditions, a quantitative analysis of the mixture can be obtained from the mass spectrum.

The propelling potentials exerted by the electrodes which force the ions from the ionization chamber into the analyzer usually are high. of the order ot several hundred volts or more, and create an electric field which may aiect the electron emission, with a consequent effect upon/the mass spectrum..

In accord-ance with my invention, thlseil'ect is minimized by interposing one or more metallic shields adjacent the electron-emitting element and between it and the electrodes. Preferably, means are provided tor maintaining a potential diilerence between the shield and the element which is independent of any potential difference between the elementy and the electrodes. For example, means may be provided for maintaining a constant potential diderence between the shield and the filament.

In the preferred form oi my invention, the metallic shield, which may comprise several sections maintained at diil'erent potentials, surrounds the electron emitting element substantially completely, i. e.I7 is disposed on all six sides of the element.

but is provided with an aperture for the emission of an electron beam. In this preferred form the electron emitting element (lament) is mounted externally of the ionization chamber, and the insulators on the leads which supply current .to the nlaxnent are protected by one or more metallic shields interposed between the insulators 'and the filament, so .that metal vaporized and emitted by the lament is deposited on the shields instead of on the insulators.

The presence o! a shield (say the shield 91) between the ionization chamber and the nlament is desirable because this shield can be used to create a space charge around the filament to protect it from the eilect of positive ions and minimize fluctuations in electron emission due to iluctuations in filament current. These results can be obtained without ailecting the energy of the electron beam entering the ionization chamber.

i' The electrode arrangement shown in this applil autres iA is a plan of the exit plate of the analyzer tube of Fig. l;

Figs. 2 and 8 are longitudinal sectional views oi' the head oi.' Fig.- 1, taken through the axis of the assembly and, respectively. perpendicular and parallel to a magnetic neid impressed on the ionization chamber and analyzer tube `by a magnet (not shown);

Fig. 4 is a section through the head of Figs. 2 and 3 taken along the line l-l of Fig. 3 of the apparatus;

Fig. 5 is a section through the illament assembly of Fig. 4 taken along the line 5 5;

Fig. 6 is a section through the filament assembly taken along the line tof Fig. 5; and

Fig. 7 is a sketch showing the box-shaped shield of the iilament assembly.

In the type ci mass spectrometer herein` described, the envelope and the elements which it encloses are normally mounted in a horizontal plane between the poles of an electromagnet (not shown).

General assembly in the envelope a nat section il at the very end thereof through u which the gas introduction conduit passes. This ilat section is in the form of a glass plate which is secured to the `envelope with wax. This wax is kept from melting during the operation of the mass spectrometer by means of a cooling fluid o which is circulated through a copper tube i5 in contact with the glass plate adjacent the wax joint. 4

The enlarged section of the envelope has three side branches. One of these is terminated in a spherical point I8 which is adaptedfor connection to a pumping line Il. The second side branch is terminated by an electrical connector Il having plug terminalsfor supplying various currents and voltages to electrodes in the ionization chamber.

In the third branch there is sealed a lead I9 which may be used as a ground connection. The second and third branches are spaced apart considerably in order to insulate the ground lead from the plug terminals since these are operated at a voltage which may be either positive or negative with respect to ground but which in either event may differ greatly from ground potential.

The end of the envelope remote from the enlarged section has a tapered ground joint surface 2l on the outside. A metal disc 2| acting as an coaxial with the ground joint and within the envelope. This rod extends through a glass seal at the end of the envelope and makes an electrical connection to the grid of an amplifying tube con- Y tained in an evacuated metal tube (not shown) which nts over the ground joint.

A heater is provided for heat treating or baking out the envelope assembly, and comprises the following layers which are wrapped successively about the semi-circular portion of the en velope betwe'en the enlarged section and the ground joint:

A layer of asbestos 3|;

ion collector is mounted on a rod or connector 22 4 A coll of Nichrome wire Il adapted to be heated by electric current: Two layers of asbestos 38: and

A layer of metal foil ll.

. In order to provide against the/accumulation of any electrical charges on the inside wall of the envelope, it is coated with a layer 3l of colloidal graphite which is maintained at ground potential through a platinum deposit ll on the wall and a spiral lead I1 which is connected to the ground lead within the envelope.

The asbestos layers serve to insulate the coil from the foil and also to distribute the heat from the coil over the surface of the glass envelope.

The foil aids in heating the assembly by reducingv outward radiation 'of heat from the coil.

The analyzer tube is centered within the glass envelope with the ald of feet Il extending outwardly from the walls of the analyzer tube and flexible curved leaf springs 4I secured to the outside of the analyzer tube. These centering springs are secured at one end rigidly to the analyzer tube and are held -in axial alignment with the tube by means of screw heads at the other end which overlapslotted parts of the spring.

Small exhaust ports I2 are drilled into the analyzer tube at numerous points throughout its length at the front and back thereof.

Slots 43 are cut into the outside circumferential edge cf the analyzer tube. Metal bailles (not shown) may be inserted into these slots to aid in the elimination of ghosts" (which may arise because of the reflection ot ion beams at the inner wall of the analyzer tube).

At the outlet end of the analyzer tube there is secured an aperture plate M on which (see Fig. 1A) a-re mounted two slidable jaws 4l, Il, the sharp edges of which denne an exit slit 41 through which ions are projected from the analyzer tube onto the collector.

At the inlet end of the analyzer tube, there is a iiange Il to which the head il is seemed by pivoted clamps.

Head, including ionization chamber Details of the head are shown in Figs. 2, 3, 4, 5, 6 and 7. By reference to these gures it will be seen that the head comprises a block Il in the form of a thick-walled metal cylinder, a quartz disc 5I secured to the inlet end of the block, a pair of pusher segments 82, 53 separated by a quartz insulator 54 and extending into the block bore 55 through the quartz disc. an electron gun 58 (Fig. 3) mounted on the quartz disc, a pair of jaws 51, 5l positioned at the outlet sideof the block to define a ilrst slit S1, a second pair of jaws 59, 60 separated from the rst pair of jaws by a Pyrex insulatingring il and dening a second slit Si, and a head mounting flange Cl spaced from and electrically connected to the second pair of jaws by means of a copper spacer 81.

The pu'sher segments are secured to the block by means of a pusher clamp 10 (which cover a Pyrex pusher locking ring 1i surrounding the two pusher segments) screwed to the block through the quartz disc. The block and iirst pair of jaws, the Pyrex insulating ring, the second pair of jaws, the copper spacer, 'andthe head mounting ange are held together by quartz links I2 each of which has an eye at one end (which is positioned over a link-stud screw 'il' which extends outwardly from the sideof the block wall) and at the other end a second eye perpendicular` to the iirst eye. The second eye is secured tothe head mounting ilange by means of spring link clips 14,

annees v Jected through a slotted insert Il in the wall ofv the block into the ionization chamber and thence through a concentric bore Il in the other side of the block onto an electron catcher Il which is supported by the quartz disc and extends into a bore It drilled part way into the block perpendicular to the bore Il. l

The block. the pusher segments, the central part of the quarts disc and the first pair of slit `iaws form a substantially enclosed space that serves as an ionization chamber.

The entire assembly is so arranged that the adJacent but spaced edges of the pusher segments, the axis oi' the electron beam. the axes oi' slits 8i andsaandtheaxisoitheexitslitoftheanalyaer tubeareallparaileltoeachotherandtolinesof force of the magnetic neld in which the entire envelope assembly is to be placed.

Electron beam source A illament el which acts as an electron source in the gun is supported by two L-shaped filament supports Il. l2 which in turn are fastened to filament posts Il, M mounted on the vquartz disc and perpendicular thereto. Changes in the position of the illament are prevented in part by means of a square quartz locking plate II. two edges of which rest against lock-ing ilaps cut into the sides oi' the ilaxnent posts. An apertured electron accelerating electrode I is mounted intermediate the illament and the slotted insert or gun barrel" l! through which electrons are to be propelled into the ionization chamber. A boxshaped shield el open at opposite ends is disposed around the illament and supported by one of the il-lament posts with the planes defined by its open ends perpendicular to the axis of the electron beam. A rear filament shield Il is supported by the other nlament post and is disposed in the 'open end of the box-shaped shield opposite the electrode Se.

The electron accelerating electrode, the boxshaped shield, and the rear nlament shield are insulated from each other. Together, they prevent metal evaporated from the filament from striking the quartz disc, for such metal strikes one or the other of the members and condenses before it reaches the quartz insulator.

An external illament shield Ill having a general L-shape is mounted with one flat side in contact with the block at the side remote from the quartz disc, and with the other side of the L generally concentric with the block. This shield thus encloses the electron gun except for an evacuation slot Iii. Thus. the apparatus is provided with an enclosed electron gun chamber Ill which may be evacuated through the slot.

Electrical elements Considered from an electrical standpoint, the

block and first jaws represent a first ion accelerat the potential of the other end oi the illament. Theouter filament shield is at the same potential as the block and hence at the same potential as the first ion accelerating electrode. The outer filament shield and the wall of the block provide an equipotent-ial surface which shields the filament from stray fields that might arise because of any high potential applied between the nrst ion-accelerating electrode and the second ionaccelerating electrode. Any residual stray. field that might leak toward the filament because of the presence o! the slot and the non-conductive insulating surface of the quartz disc at the bottom o! the electron gun chamber is prevented from doing so by the presence of the potentials existing on the box-shaped guard disposed around the diamant and comprising the electron-accelerating electrode. the open-ended box-shaped shield and the rear diamant shield.

Operateur A gas mixture. say a mixture of hydrocarbons. may be analyzed' by introducing it at low pressure -lnto the ionization chamber through the sample inlet conduit l2. Thus the gas enters the ionization chamber in the head il through a passageway between the two pusher segments Il. Il.

In the ionization chamber, the molecules oi gas are bombarded by an electron beam originating at the filament of the electron gun and passing through the gun barrel Il. An electrical potential is impressed between the pusher electrode (i. e.. the interior ends of the two pusher segments) and the jaws I1, 5l which deilne the slit S1. This potential pushes the ions through the slit Si as a heterogeneous ion beam. This beam is further .accelerated by a potential impressed between the jaws deilning the slit Si and the jaws 59. te which denne the slit Sz. The two slits act as aicollimator and the heterogeneous ion beam or 'ribbon is projected through the slit S2 into the analyzer tube. In the analyzer tube the ions are acted upon by a magnetic field which separates them according to their specific masses into a plurality of curved homogeneous ion beams which diverge from each other and are focused at dierent points within the analyzer, for example on the exit plate. By varying the magnetic field the radii of the curving ion beams may be changed so that the beams are swept successively across the exit slit 41 of the instrument. In this fashion the several beams may be brought to focus successively on the ion collector and there discharged. The currents thus formed constitute the mass spectrum of the gas mixture and, if amplified and recorded, produce a mass spectrogram.

The propelling potential which is impressed between the pusher electrodes 'and the electrodes Il, Il (which define the slit Si) is relatively high,

and the potential impressed between the slits Si. Sz (i. e.'. between the ion accelerating electrodes) maybe very high-of the order of several hundredyvolts. These potentials create an electric field which may aifect the emission of electrons by the electron gun, with a serious eilect upon the mass spectrum. The effect of electric ilelds created by the propelling potentials in the head of the mass spectrometer is minimized by the metallic shields which are disposed around the ion emitting illament. Thus in the apparatus illustrated, the lament il is substantially surrounded by a metallic shield comprising the box-shaped shield Il. the rear shield Il and the electron accelerating electrode II, which has the aperture as one end of the filament. and thebox shield'ia a,444,ces

through which the ion is pruiected.` The. leads, i. e., the supportingjposts which hold the filament are insulated by the quarta plate and this plate is protected by the shields, so that metal vaporized and emitted by the lament is deposited on the shields instead ofon the insulator.

'I claim:

cent thereto, one of these elements being provided i with an aperture for the emission of the electron beam, and means for maintaining another of these elements at the same potential as one end o! the electron emitting element.

2. In a mass spectrometer provided with an ionization chamber, means for admitting molecules to be ionized into the chamber, an electron emitting element, means for propelling the emitted electrons against molecules in the chamber to convert the molecules into ions, and electrodes disposed in the neighborhood of the electron emitting element and the ionization chamber for impressing a propelling potential on the ions `thus formed, the combination which comprises a separate chamber in which the electron emitting element is mounted, a conduit connecting the separate chamber to the ionization `chamber to permit the emitted electrons to enter the ionization chamber, an ion outlet from the ionization chamber comprising an aperture in one oi the electrodes, a shield at the side oi the electronemitting element opposite the last-mentioned electrode, and means for maintaining said shield and the last-mentioned electrode at substantially the same potential.

3. In a mass spectrometer provided with an ionization chamber, means for 'admitting molecules to be ionized into the chamber, an electron emitting lament, means forpropelling the admitted ,electrons against the molecules in the chamber to convert the molecules into ions, and electrodes disposed in the neighborhood ot the electron emitting element and the ionization chamber for impressing a propelling' potential on the ions thus formed, the combination comprising a six sided metal shield substantially enclosing the electron emitting illament, the front side o! the shield being part oi the means for propelling the emitted electrons and being insulated from the other sides oi the shield', the rear side of said shield opposite said front side also being insulated from the remaining sides of the shield, means for maintaining the rearv side at the potential of one 'end o! the iilament loi! the ilament.

8 and means' tor maintaining the remaining sides at the potential ci the other 4. In a massspectrometer provided with ionization chamber, means for cules to be ionized into the chamber, an 4electron emitting iilament, means for propelling emitted electrons against molecules in thechamber to convert the molecules into ions and electrodes dilposed in the neighborhood of the electron emitting filament and the ionization chamber for impressing a propelling potential on the ions thus formed, the* combination which comprises a four sided box disposed around the illament and op'an at'opposite ends, an electrode for propelling the emitted electrons so disposed as to substantially close one end of the box and being insulated there from, a shield substantially closing the opposite end of the box andbeing insulated therefrom, means for maintaining the shield at the potential of one end oi the filament and means formamtaining the box at the potential of the other end Aminou) W. wAsHBUaN.

REFERENCES CITED The following references are of record in 'th file of this patent: f

I UNITED STATES PATENTS Number Name Date 2,221,467 Bleakney Nov. 12,- 1940 2,331,189 Hippie Oct. 5, 1943 OTHER .REFERENCES pp. 496-501 of Physical Review; vol. 40, 2nd series.

May 15, 1932. (Photostat copy in Divisionv 3d, U; S. Patent Ofilce.) 'I3-51 Mass. Spec.

Technical Publication, A Mass Spectrometer for Routine Abundance Measurements," by Alfred O. Nier, pp. 212-216, VReview of Scientiiic Instruments, vol. 11, No. '1, July 1940. (Photostat copy in Division 36. U. S. Patent Oiiice.) 'I3-51 Mass. Spec.

Technical Publication, .Primary and Secondary Products of Ionization in Hydrogen, by HJ D. Smith, pp. 452-459 of Physical Review, v01. 3l,

1925. (Photostat copy in Division 3 8, U. S. Patent Oiilcc.) 'I3-51 Mass. Spec. Technical Publication, An Experimental llnsl Spectrometer," by N. D. Coggeshall et al., pp.

1223-129, Review of Scientific Instruments, vol. 14, No. 5, May 1943. (Photostat copy in Div. 36. U. B. Patent Ofiice.) 'Z3-51 Mass. Spec. 

